Large conductance Voltage and Ca2+ activated K+ channels (BK) are membrane proteins that play a fundamental role in controlling smooth muscle tone and neuronal excitability. In most of the tissues, they form complexes composed by the pore-forming alpha subunit and by regulatory subunits. Similarly the other voltage dependent ion channels, BK posses a voltage sensor that is mainly represented by the S4 transmembrane segment. Changes in potential across the membrane displace the voltage sensor, producing a conformational change of the protein. Eventually, for adequate depolarizations, the consequent conformational change brings the channel into a state that allows ion conduction. Very little is known about structures that regulate the opening and closing of BK channels, and no information exists about the dynamical rearrangements produced in BK channels by changes in the membrane potential. One of the aims of this project is to investigate the structural changes underlying the operation of BK channel. Conformational changes of both a and b subunits will be assessed by residue specific fluorescent labeling of the channel protein. The short-term objective is to obtain a more realistic view of BK protein by identifying regions of motion that underlie voltage sensing, and that are couple to activation, inactivation and deactivation, and regions of relative staticity, involved in other channel functions. In addition the role of charged residues in pore region will be investigated. BK channel activators are now under close investigation for treatment of urinary incontinence and as stroke neuroprotectant. This study will contribute to set a framework for the design of new therapeutic agents or for the amelioration of the one already adopted by the medical practice.